Antenna arrays have been widely used in wireless mobile networks for directional signal transmission and reception with an increased gain compared to an omni-directional antenna. The increased gain translates into a higher cell density and data throughput. An antenna array needs to be calibrated across its sub-array paths to remove any linear phase and/or amplitude distortions in these paths. If the transmission beam pattern is out of phase or otherwise phase-distorted, the signal transmitted by a base station at normal transmission power may not be correctly received and decoded by a wireless device. To compensate for the phase distortions, the base station may transmit data at a higher power level. However, increasing the transmission power acts as a load to the system, causing a reduction to the power that can be allocated to other wireless devices. In addition, the signal transmitted at higher power may interfere with other wireless devices, causing a reduction in signal quality.
One existing method for antenna array calibration uses special calibration signals injected into the transmit path of the base station. The special calibration signals may interrupt and/or degrade the normal outbound traffic signals, which can negatively impact the network capacity and data throughput. Additionally, there are currently a wide variety of base stations that have different system configurations to support multiple communications standards and multiple carriers. The use of the special calibration signals by these base stations may result in standards non-compliance and/or violate regulatory requirements.
Further, phase variations, such as distortions, may be caused by impairments introduced by use of separate timing or clock components in each of different transmit chains, for example different radio frequency local oscillators, voltage controlled crystal oscillators, clock recovery phased locked loops, etc. Each transmit chain lies in a transmit path that extends from an input signal port to a subarray element. The rapid nature of these impairments require correction at a fast rate, a requirement conventional solutions cannot easily meet because of their complexity and processing requirements.
Problems with known solutions to the problem of correcting impairment in transmit paths of a radio transmitter include a requirement to introduce dithering in highly correlated signals, failure of the known solutions to linearly scale with the number of elements of the antenna arrays, need for a special calibration port built into the radio unit of the transmitter, and sharing the receiver in the radio unit to process feedback signals used to estimate impairment.